Re: [core] draft-ietf-core-observe-05 - "obs"

Zach Shelby <zach@sensinode.com> Fri, 20 April 2012 15:38 UTC

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From: Zach Shelby <zach@sensinode.com>
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Date: Fri, 20 Apr 2012 18:38:37 +0300
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To: Klaus Hartke <hartke@tzi.org>
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Subject: Re: [core] draft-ietf-core-observe-05 - "obs"
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Hi,

I would like to keep the 'obs' link attribute. From the experience deploying products and systems using CoAP so far, we have actually found this attribute useful. Two reasons:

1. Although in theory the protocol still works through trial and error as you describe below, it is useful for a CoAP client to plan its behavior before making a request.

2. Knowledge of the Observability of a resource is useful at the application layer, where some M2M application needs to make use of these resources. The behavior of dealing with a polling-only resource vs. an observable resource is pretty different, and an observable resource might result in a different graphical representation to the user which needs to be known already during discovery.

I like the philosophy otherwise for finding things to take away, but not this one please.

Zach

On Apr 17, 2012, at 3:14 AM, Klaus Hartke wrote:

> In the spirit of "In protocol design, perfection has been reached not
> when there is nothing left to add, but when there is nothing left to
> take away" [1]:
> 
> Do we really need the "obs" link attribute?
> 
> Some thoughts:
> 
> * If a client wants to have a fresh representation of a resource over
> a period of time, it can include the Observe option in its request. If
> the server does not support -observe, the client can poll the resource
> to achieve its goal.
> 
> * If a client for whatever reason only wants to have a fresh
> representation of a resource over a period of time if the server
> supports -observe, it can include Observe option in its request and
> not poll if the if the server does not support -observe.
> 
> * If a client wants a single snapshot representation of a resource, it
> can omit the Observe option from its request.
> 
> Under what circumstances can a client not be sure if it wants to have
> a fresh representation of a resource over a period of time, so a hint
> from the server is needed?
> 
> 
> Klaus
> 
> 
> [1] http://tools.ietf.org/html/rfc1925
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-- 
Zach Shelby, Chief Nerd, Sensinode Ltd.
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